The integrated process rates (IPRs) estimated by the Eta-CMAQ model at grid cells along the trajectory of the air mass transport path were analyzed to quantitatively investigate the relative importance of physical and chemical processes for O-3 formation and evolution over the northeastern U.S. during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) period. The Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model is used to determine the back trajectory of air masses reaching the northeast by linking a downwind receptor to upwind source areas. The process analysis is applied to a high O-3 episode occurring on July 22, 2004 at three selected sites in the northeastern U.S. The process analysis at the location of the site shows that during the daytime, the O-3 concentrations in the surface layer are mainly enhanced by the vertical diffusion of O-3-rich air from aloft, followed by horizontal advection (HADV) and chemical production (CHEM), whereas dry deposition (DDEP) and vertical advection (ZADV) mainly deplete O-3 concentrations at the sites of Valley Central (VC), NY and Castle Spring (CS), NH. By integrating the effects of each process over the depth of the daytime planetary boundary layer (PBL), it was found that at the VC site, CHEM and HADV contributed about 53% and 41%, respectively, to O-3 levels within the PBL. This confirms the significance of regional transport of O-3 froth the industrialized areas into the Northeast. On the other hand, the process analysis results for O-3 formation in moving air masses indicate that on July 22, large chemical production of O-3 along the transport path over the polluted urban regions leads to significant increase in O-3 in the air mass reaching the VC site, whereas the low chemical production of O-3 along the transport path over the low emission regions leads to the low O-3 concentration at the site of Belleayre Mountain (BM), NY. The dramatic buildup of O-3 concentration from 50 ppb to 102 ppb in the air masses before reaching the VC site after 12:00 EST on 7/22 indicates the significant impact of pollution from the northeastern urban corridor at this site. On the basis of the results at the CS site, it was found that high NOx emissions along the transport path led to large chemical production of O-3 in the air mass reaching the CS site on July 22. In contrast, the low chemical production of O-3 associated with low emission (relatively clean conditions) along the transport path over the northern portions of the domain is responsible for the low O-3 concentration at the CS site on July 26. (C) 2008 Elsevier Ltd. All rights reserved.
